Conneg with intermediaries

Folks,

I'd like to explore the an approach changing SMTP-based Conneg. There is a
very basic concern that an intermediate SMTP relay will make arbitrary
changes to the sender's content.

How can the sender "authorize" an intermediary to make changes using CONNEG?

How can the sender tell the intermediary which changes are authorized and
which ones are not?

One idea is for the sender to added its own list of CONNEG features to the
message. Because the list is attached to the message, intermediaries are
authorized to make changes. The list specifies the features ("capabilities")
that are authorized for changing. An intermediary MUST NOT make content
changes for any feature not listed by the sender.


----- So the scenario for doing SMTP Conneg has the following:

A message travels through a sequence of SMTP relays.  Eventually it reaches
an SMTP transition in which the receiving SMTP server announces that it
supports CONNEG.

The sending SMTP relay also supports the feature.  But is it authorized to
use it?

The message contains a Conneg list that was provided by the sender.  (Let's
not talk about how this is attached to the message.  We can consider that
after we agree on the basics of this authorization mechanism.)

The fact that the Sender's Conneg list is present means the SMTP client
relay is authorized to make transforms.

The SMTP client relay does a logical intersection between the list provided
by the sender and the client relay's own list of capabilities that it
supports.

Assuming that the intersection results in a non-zero set of authorized
transforms, the client relay then queries the receiver for an addressee
and the receiving SMTP server returns the CONNEG capabilities for that
addressee.

The client relay performs another logical intersection between its own
conneg list and the list from the receiving SMTP server.

It now has a list of content forms that are authorized by the sender and
supported by the receiver and that the intermediary can support
transforming.

----- end of scenario

Is this acceptable?  If yes then I will try to specify changes to the SMTP
Conneg draft.

d/
-- 
 Dave Crocker  <mailto:[email protected]>
 TribalWise <http://www.tribalwise.com>
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